The present invention relates to cellular communications systems, and more particularly to methods and apparatuses for tracking cells to facilitate cell-reselection in a cellular communication system
A user equipment (UE) operating in a cellular system such as a system compliant with the Global System for Mobile Communications (GSM) selects the most suitable cell to camp on when it is first powered up. (As used herein, the terms “cell” and the base station or equivalent network node that serves that cell are used synonymously.) Once a UE is camped on a cell and is not engaged on an ongoing call, it monitors its neighboring cells for a possible cell reselection. This procedure is periodically performed on an ongoing basis in order to ensure that the UE is camped on the most suitable cell in its vicinity. There are other reasons for cell reselection but they are not essential for understanding the present invention.
FIG. 1 illustrates an exemplary arrangement in which these various procedures are performed. (As used herein, the term “exemplary” means “serving as an illustration”.) A cellular communications system 100 includes a number of cells, including a cell 101 that serves a UE 103. Neighbor cells of the UE 103 include the neighbor cells 105, 107, 109, 111, 113, 115, 117, and 119. The UE 103 is camped on the cell 101, and monitors some group of the neighbor cells 105, 107, 111, 113, 115, 117, 119 for a possible cell reselection.
According to the 3rd Generation Partnership Project (3GPP) specification (in particular, 3GPP specification 45.008 chapter 6), a UE operating in GSM mode, while in idle mode, monitors cells by performing measurements on, and reading relevant Broadcast Control Channel (BCCH) data from, the UE's serving cell and the six strongest neighboring cells. By the term “six strongest” is meant the six neighbor cells that have the highest measured received signal level (called RXLEV hereafter). How these measurements are performed and averaged is well defined and known to those of ordinary skill in the art. Therefore, there is no need to describe these procedures in further detail in this document.
With respect to monitoring the neighbor cells, it is conventional for the following activities to be performed during each occurrence of an idle mode state within the UE:                RXLEV measurements are repeatedly made on neighboring cells, and from these measurements the six strongest neighbors are identified.        For each of the six strongest neighbor cells, in order to ensure that the cell currently being tracked is the intended cell, Base Station Identification Code (BSIC) checks are made, including checking the synchronization of the frequency channel (FCH) and the synchronization channel (SCH). The standards require the UE to attempt to check the BSIC for each of the six strongest cells at least once every 30 seconds, in order to confirm that it has not inadvertently started monitoring the strength of a different cell.        For each of the six strongest neighbor cells, the Broadcast Control Channel (BCCH) is read in order to capture relevant data that is needed for cell reselection. If the serving cell does not provide all of the parameters affecting cell reselection, then the specifications require the UE to capture relevant BCCH data for each of the six strongest neighbor cells at least once every five minutes. Additionally, if the UE lacks relevant information for a neighbor cell that has newly been identified as being among the six strongest neighbor cells, then the UE is required to decode the BCCH data for that newly identified cell within 30 seconds.        
It is desirable to enable UEs to achieve desired neighbor cell monitoring performance goals in as efficient a manner as possible.